lib/ovs-atomic: Add ovs_refcount_unref_relaxed(), ovs_refcount_try_ref_rcu().

lib/ovs-atomic: Add ovs_refcount_unref_relaxed(), ovs_refcount_try_ref_rcu().

When a reference counted object is also RCU protected the deletion of
the object's memory is always postponed.  This allows
memory_order_relaxed to be used also for unreferencing, as RCU
quiescing provides a full memory barrier (it has to, or otherwise
there could be lingering accesses to objects after they are recycled).

Also, when access to the reference counted object is protected via a
mutex or a lock, the locking primitives provide the required memory
barrier functionality.

Also, add ovs_refcount_try_ref_rcu(), which takes a reference only if
the refcount is non-zero and returns true if a reference was taken,
false otherwise.  This can be used in combined RCU/refcount scenarios
where we have an RCU protected reference to an refcounted object, but
which may be unref'ed at any time.  If ovs_refcount_try_ref_rcu()
fails, the object may still be safely used until the current thread
quiesces.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>